The invention relates to emissions control for a boiler and particularly to suppressing emissions of nitrogen oxides in flue gasses generated by a boiler.
Flue gas generated from combustion of fossil fuels such as oil, gas and coal typically include airborne nitrogen oxides (NOx). NOx are pollutants and subject to increasingly strict governmental regulations to protect the atmospheric environment. There is a long felt need for systems to reduce NOx in flue gas.
To reduce the nitrogen oxides in flue gas, various systems are deployed in the flue gas passages of the boiler. These conventional systems include selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR) which both react reagents with the flue gas. The reagents, such as ammonia and urea, react with nitrogen oxides in the flue gas to remove NOx from the flue gas.
SNCR involves the injection of a reagent in hot flue gas. The heat in the flue gas promotes a chemical reaction between the reagent and NOx to reduce the NOx in the gas. SCR also involves a reagent reacting with flue gas, but is performed on cooler flue gas. As gas cools, the flue gas passes through the SCR system which has a catalyst to promote the chemical reaction between the reagent and NOx and thereby further reduce NOx in the gas. The catalyst is typically on a supporting structure positioned in the path of the flue gas. In a SNCR/SCR coupling system, the reagent used for the SCR system can be a portion of the reagent introduced by the SNCR system. The portion is the remaining reagent that did not react with the NOx in the SNCR system. The remaining reagent flows with the flue gas to the SCR system. The remaining reagent is often referred to as “ammonia slip” from the SNCR system.
A uniform distribution of the reagent remaining in the flue gas is desirable as the gas passes over the catalyst in the SCR system. Structural or mechanical mixing devices, such as arrays of plates, have been positioned in the flue gas upstream of the SCR system to ensure that the reagent is uniformly distributed in the flue gas.